5570: Advanced Unix Programming

Midterm review

Note:

C and related features

1. You should be able to program in C.

2. Makefiles: you should know the material discussed in class, and in the examples. In particular, (i) you should be able to create targets and dependencies, first compiling object files, and then linking them together. (ii) You should know how to use macros, for example define CC as gcc. You need not learn about inference rules.

3. Compiler: You should know the -g, -c, -o, -O[n], -l, -I, -L, -Wall, -ansi, -pedantic, and -D options to gcc. (You need not learn about -pg and feedback optimization for the exam; however, you may find these useful in real life.)

4. Header files: You should know what pre-processing is, how macros are preprocessed, about guards, errors that can happened due to incorrect macro use, C linkage in C++ programs, incorrect header files use (for example, defining global variables or functions in the header file). (You may also want to learn about disadvantages of pre-processing, and alternatives to its features. This is not be required for the exam, but may be useful in real life.)

5. You should be able to implement features based on HW 1, such as reading a line at a time, command line arguments (including converting the command line argument strings to int and long), and obtaining the value of an environment variable.

6. Some useful functions, such as sprintf, sscanf, strcpy, strcmp, atoi, atol, system, malloc, free, etc.

7. Portability: Problems that can occur due to different sizes of integer types, little-endian vs big-endian, how you can find out if a machine is little endian or big endian, POSIX standardization and _POSIX_SOURCE macro.

Introductory material on Unix API

1. How to set environment variable on the command line, in Bourne and C shells, how to get the value of an environment variable from a program, using getenv().

2. Exit status, the difference between exit and _exit.

3. Process ID, getpid, getppid, realize that the ps command presents the process IDs too.

4. User ID, real versus effective user ID, saved-setUID, how these are set on exec (you can ignore group ID), getuid/setuid (you can ignore the case of processes running with super-user privilege).

5. errno

6. Note: You need not memorize which header file is to be used for which system call!

Unix File system

1. Concepts of file, directory, link, file name, inode, and symbolic link, the technique of 'unlink'ing a temporary file immediately after creating it (AUP page 140).

2. Some questions: What is the difference between a soft link and a hard link? Does the i-node store a file name? What operations are involved in moving a file? Is the i-node deleted if a soft link is deleted? Is an i-node always deleted immediately after the last link to it has been deleted? Do names belong to links or to files? What happens on renaming a file? When can a file be deleted?

3. File access permission, ownership of new files and directories (ignore the group ID issue).

File I/O

1. File descriptors and open file description, which file descriptors are used for stdin, stdout, and stderr respectively, were are the file offsets and the file sizes stored, how does this affect appending to files, can multiple file descriptors point to the same open file description, can multiple processes point to the same open file description, can multiple open file descriptions point to the same inode, order of file descriptors on opening files.

2. Some questions: What is the difference between a file descriptor and an open file description? Is there a unique open file description for each file? Is there a unique open file description per process, for each file opened by that process? Can multiple processes point to the same open file description? Are there distinct offsets available for reading and writing, or is there only one file offset? Does the file offset belong to the inode, or to an open file description, or to the file descriptor? If two different open file descriptions point to the same file, then are their offsets kept equal?

Process management

1. fork, what does it return, how do you distinguish between parent and child, what is duplicated on the child, what happens to file descriptors, is there any relative order of execution, is the memory of the parent and child shared, or is it just copied once (and then remains distinct), sec 8.3 and fig 8.1 are particularly important.

2. exit and wait, how can you determine the exit status and whether the child exited normally, will code after the exec statement be executed if the call is successful, what if the call was not successful, what happens to the process's data after calling exec, what about the file descriptors, learn about the FD_CLOEXEC flag.

3. Idea behind the exec family of commands, execv.

4. Some questions: What happens to the file descriptor after forking? If one of the processes closes one end of the pipe, will the other process still be able to use it? Similarly, what happens after an exec? What is a race condition?

Pipe

1. pipe, is a pipe available to the child too, if you fork and then create a pipe, then will that child have access to the pipe, is there a limit to the pipe size, implementing pipe in a shell,

2. Some questions: Which command is used to create a pipe? What is the argument to the pipe() command? Can we pass an int * as argument, without allocating memory for it -- why or why not? Which end is used to write? Which end is used to read?

Signals

1. What is a signal, what are the possible signal dispositions (catch, ignore, default action), signal(), which signal does Control-C send, can SIGKILL be caught or ignored, can SIGINT be caught or ignored, what happens after a signal is caught, SIG_IGN and SIG_DFL macros, learn that the signal function can cause the handler to be erased after each invocation, what can go wrong with this scheme?, what happens to the signal masks and dispositions after exec, what happens in the child (after the parent forked it), what happens to pending signals -- for forked process and exec-ed programs.

2. sigemptyset, sigfillset, sigaddset, sigdelset, sigismember, sigprocmask, sigaction, sigpending, SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK, sigprocmask, what is the difference between the effect of SIG_BLOCK and SIG_SETMASK, does the old signal mask give the signals that are blocked, or those that are unblocked, sigset_t type.

3. sigaction, struct sigaction and its fields: sa_handler, sa_mask, sa_flags, do not set the sa_sigaction field to 0!, semantics of sigaction.

4. kill, what does it do, can it be used to send signals only to related processes, what does the alarm system call do, note the difference between the latest POSIX definition, and what the book mentions about the POSIX definition when pid==-1.

5. Some questions: What is a signal? Why is the ANSI signal function considered unreliable? What happens after a signal is caught? What is the typical effect of an uncaught signal? Do identical blocked signals have to be queued according to POSIX, and are they usually queued? Is the type of signal caught included in the signal mask for the duration of the the signal handler by default (with sigaction), Is the signal handler reset to the default action, after each invocation of the signal handler (with signal? with sigaction?)? What happens to "slow" system calls that are interrupted by a signal? (Read the POSIX standard, in particular, the SA_RESTART flag under sigaction. You need not use this flag, but it will help you learn about what the standard requires.) What do the terms 'generate', 'deliver', and 'pending' mean, in the context of signals. What does kill do? What arguments should be provided to the 'kill' function? Can you "kill" someone else's processes, normally? Can you kill any of your processes, or should the process sending the signal be a relative of a process it tries to 'kill'? What does the 'alarm' system call do? What happens if a signal is caught in the 'read' system call, when you are trying to read from the terminal? What if you were reading from disk?

Be prepared for questions related to HW2 too.

Look at the example programs.

Review quiz questions, and other issues I mentioned in class as important.